1. Field of the Invention
This invention relates to solder mask coatings having improved performance characteristics, and, more particularly it is concerned with aqueous alkaline developable, UV curable compositions for making flexible solder mask coatings having tenting capabilities.
2. Description of the Prior Art
A number of U.S. patents disclose photopolymerizable prepolymers and compositions for use as solder masks in printed circuit boards, including U.S. Pat. No. 3,660,088; 3,753,720; 3,883,352; 3,887,450; 4,199,163; 4,361,640; 4,422,914; 4,436,806; 4,458,007; 4,481,281; 4,506,004; and 4,508,916. However, none of these UV curable compounds and compositions are entirely satisfactory for use as a commercial photoimageable solder mask. A feature of a preferred photoimageable solder mask coating is an ability to be developed with an aqueous, slightly alkaline solution, thereby avoiding the use of organic solvents in the developing step. The desired solder mask coatings also should exhibit flexibility, heat and chemical resistance, surface hardness, abrasion resistance, adhesion to the underlying metal of the printed circuit board, and a high cure depth at low dosages. Particularly sought after are solder masks which exhibit a rapid curing rate and tenting capabilities. These stringent requirements imply that the UV curable compound and compositions must be carefully formulated in order that the solder mask can achieve commercial acceptability.
For example, in Rendulic, U.S. Pat. No. 4,436,806, there is described a method and apparatus for making a printed circuit board by imaging a liquid polymer which has been coated onto the board by roller coating. In this process, imaging is carried out while the polymer is wet, the phototool being positioned in a close relationship with the coated board. The liquid polymer compositions disclosed by Rendulic for use in this process are urethane acrylates having a polyether or polyester chain extending unit. These compositions have a very low viscosity which is desirable for forming coatings with a roller coater. However, these compositions have not shown tenting capabilities.
In contrast to the imaging technique of the abovementioned patent, Sullivan, in U.S. Pat. No. 4,506,004, describes a contact method of imaging a liquid solder mask coating on a printed circuit board. In this method, the UV curable solder mask coating is screen printed to a given thickness, and imaged by exposure with suitable UV energy. The desired characteristics of good electrical performance and excellent printing resolution are the attributes of the Sullivan system. The requirements of a suitable liquid photopolymerizable composition for use in this process include an ability to form a smooth, flexible coating which can impart chemical and heat resistance to the solder mask. In addition, the coating must exhibit excellent adhesion to the board, an ability to be cured to a desired depth with a minimum of UV energy, excellent electrical insulation resistance, and tenting capabilities.
Tenting describes the ability of solder masks to cover, bridge or span an unsupported area, such as tooling holes or plated-through holes. This property is particularly sought after where the solder mask is used to produce an all copper, plated-through hole printed circuit board which is processed by panel plating to provide the desired amount of copper in the hole and on the panel surface. Such panel then is etched using the solder mask coating to protect the holes and conductor pattern. Some multi-layer boards also are made by the tent and etch process.
The tenting ability of the solder mask coating can be used to protect certain holes in a pattern plating process from receiving additional plating. This solder mask tent keeps additional electroplating material out of the non-plated-through holes.
These and other requirements of a commercial liquid photopolymerizable composition for making liquid photoimageable solder masks to augment or replace dry film solder masks are described in an article in Circuits Manufacturing, entitled "Liquids Make a Splash," pages 24-33 (January, 1987). In particular, it is stated in this article that liquid systems cause tenting problems because liquid tents over unsupported areas break when solder is applied.
Liquid solder masks also fail because uncured liquid present inside the holes is affected by gas pressure generated during baking, or when subjected to hot air leveling, that is, solder dipping operations under compressed hot air. These effects are particularly acute if the hole is 10-60 mils in diameter.
Accordingly, it is an object of this invention to provide a UV curable composition for making a solder mask having advantageous performance characteristics.
Another object of this invention is to provide a UV curable composition, which includes a mixture of UV curable compounds, for use as a solder mask, and, particularly, a composition which can be developed rapidly in an aqueous, slightly alkaline solution.
A particularly object herein is to provide a flexible solder mask coating which exhibits tenting capabilities.
Still another object herein is to provide a solder mask coating which can be rapidly cured, preferably at an energy level of less than 0.5 Joules per sq. cm., to provide a cure depth of at least 18 mils.
Another object of this invention is to provide a UV curable composition which can be coated, preferably by screen printing techniques, to form a smooth, uniform, glossy, flexible, coating up to 3 mil thickness.
Yet another object of the invention is to provide a solder mask composition which is thermally stable without gelation for at least six months.
A further object herein is to provide a flexible solder mask coating having substantially 100% adhesion to the underlying metal of the printed circuit board, particularly on copper and tin-lead.
Another object herein is to provide a solder mask product which exhibits good surface hardness, and abrasion, electrical and organic solvent resistance.
Still another object is to provide a solder mask coating having excellent heat resistance so that it does not blister after 20 seconds at 285.degree. C.
A specific object herein is to provide a solder mask composition which is particularly adapted for use in the method and apparatus of making printed circuit boards described in U.S. Patent 4,506,004.
These and other objects and features of the invention will be made apparent from the following description of the invention.